Examples of all operation codes are in the ILE RPG Reference manual. That’s where we look up every question like this.
I use this routine for lookup requests on entry fields. The program called is a popup window of valid entries.
d POS S 3 0
C EVAL POS = %SCAN('?': ECUST )
C IF POS <> 0
c call 'RVEH600W7'
C PARM ECUST